The goal of this proposal is to elucidate the interplay between the innate immune system and marginal zone (MZ) B cells in humans. MZ B cells interface with the circulation and rapidly produce protective antibodies to blood-born pathogens through a T cell- independent pathway that remains poorly understood. We recently found that the human spleen contains a novel subset of B cell helper neutrophils (NBH cells) with a phenotype and gene expression profile distinct from those of conventional neutrophils (NC cells) present in our circulation. In this proposal, we hypothesize that NC cells undergo splenic reprogramming into NBH cells through an IL-10-dependent pathway involving perifollicular sinusoidal endothelial cells. We contend that sinusoidal endothelial cells release NC cell- attracting chemokines and IL-10 upon sensing blood-born bacteria through Toll-like receptors. We also argue that IL-10 from sinusoidal endothelial cells stimulates NC cells to differentiate into NBH cells with powerful MZ B cell-stimulating activity. The following three aims will be pursued. Aim 1 is to determine the mechanisms by which splenic sinusoidal endothelial cells induce reprogramming of NC cells into NBH cells upon sensing bacteria through Toll-like receptors. Aim 2 is to elucidate the mechanisms by which NBH cells induce IgM production, IgG and IgA class switching, and plasma cell differentiation in MZ B cells. Aim 3 is to evaluate the mechanisms by which NBH cells induce V(D)J gene somatic hypermutation and high-affinity antibody production in MZ B cells. These studies will dissect the interplay between neutrophils and MZ B cells by taking advantage of unique cells and tissues from healthy individuals and patients with rare primary immunodeficiencies. Results from these studies may lead to the identification of novel neutrophil-targeting strategies that may enhance vaccine-induced antibody responses by harnessing the B cell helper activity of NBH cells.